Inquiry question: How important is it for genetic material to be replicated exactly?
Students model the processes involved in cell replication, including but not limited to mitosis and meiosis
Table of Contents
Meiosis and the Cell Cycle
Like mitosis, meiosis represents one phase in the cell cycle – the M phase. However, there are significant major differences between the two types of cell divisions:
- Meiosis generally occurs in multicellular eukaryotes, although unicellular eukaryotes such as yeast do divide meiotically.
- In multicellular eukaryotes, meiosis occurs only in the reproductive tissues that give rise to the germline. All other cells (somatic) divide mitotically.
- DNA recombination (exchange of portions of DNA) between homologous chromosomes occurs in meiosis but not mitosis
- In mitosis, the genetic makeup of the daughter cells differs from that of the parental cells – this is not the case with mitosis.
- In mitotic divisions, the M phase consists of a single round of nuclear division, while in meiosis, two rounds of nuclear divisions occur.
Ploidy and Homologous chromosomes
In genetics, ploidy refers to the number of sets of chromosomes a species normally has. Many eukaryotic species are diploid – they have two sets of chromosomes. Some are haploid (one set of chromosomes). Species with more than 3 sets of chromosomes are said to be polyploid, although specific terms are used to describe the number of sets of chromosomes they possess.
| Number of sets of chromosomes | Term |
|---|---|
| 3 | triploid |
| 4 | tetraploid |
| 5 | pentaploid |
| 6 | hexaploid |
| 7 | septaploid |
| 8 | octoploid |
| 9 | nonaploid |
| 10 | decaploid |
In diploid organisms, one set of chromosomes comes from each parent. When the chromosomes of a diploid cell are arranged in terms of their appearance (called a karyotype), it is evident that chromosomes can be paired based on their physical characteristics, as shown in the following figure.
| Chromosome | Length/cm | Length/base pairs |
|---|---|---|
| 21 | 1.6 | 45,090,682 |
| 22 | 1.7 | 51,324,926 |
| 19 | 2.0 | 61,707,364 |
| Y | 2.0 | 62,460,029 |
| 20 | 2.1 | 66,210,255 |
| 18 | 2.7 | 80,542,538 |
| 17 | 2.8 | 84,276,897 |
| 16 | 3.1 | 96,330,374 |
| 15 | 3.5 | 99,753,195 |
| 14 | 3.6 | 101,161,492 |
| 13 | 3.9 | 113,566,686 |
| 12 | 4.5 | 133,324,548 |
| 10 | 4.6 | 134,758,134 |
| 11 | 4.6 | 135,127,769 |
| 8 | 5.0 | 146,259,331 |
| 9 | 4.8 | 150,617,247 |
| X | 5.3 | 154,259,566 |
| 7 | 5.4 | 160,567,428 |
| 6 | 5.8 | 172,126,628 |
| 5 | 6.2 | 182,045,439 |
| 4 | 6.5 | 193,574,945 |
| 3 | 6.7 | 201,105,948 |
| 2 | 8.3 | 242,696,752 |
| 1 | 8.5 | 248,387,328 |
Thus a pair of chromosomes with similar physical characteristics is referred to as homologous chromosomes (recall that each chromosome of a homologous pair comes from a biological parent). The shared characteristics between a homologous pair of chromosomes include:
- similar lengths
- similar gene positions
- similar locations of the centromeres
The following figure illustrates the shared characteristics of human homologous chromosomes.
An overview of meiosis
Cells enter meiosis after duplicating their genome
Meiosis I
Meiosis I differs from mitosis I in several significant ways.
Prophase I
As in mitosis, chromosome condensation, centrosome duplication, spindle fibre formation, and nuclear envelope breakdown occur. However, the chromosomal events in prophase I are different to mitotic prophase.
Leptotene (“thin threads”)
Leptotene is the phase during which chromosomal condensation occurs
Zygotene (“paired threads”)
Homologous chromosomes are lined up next to each other, in close proximity (~100 nm). This pairing of homologous chromosomes is also called synapsis. The homologous chromosome pairs are held together by a protein complex called the synaptonemal complex.
Pachytene (“thick threads”)
At various points along their lengths, non-sister chromatids undergo a process known as recombination (also referred to as crossing over). During recombination, the DNAs of the sister chromatids go through double-stranded breaks and recombine. Thus, segments of DNA are exchanged between the non-sister chromatids. Those break (recombination) points are called chiasma (singular, chiasmata).
Diplotene (“two threads”)
The synaptonemal complex holding the homologous chromosome pairs breakdown, and the chromosome pairs move slightly apart. However, the non-sister chromatids are still joined at the chiasma.
Diakinesis
Chromosomal condensation is complete. Visually, the chromosomes appear as tetrads and are attached to spindle fibres. The diplotene stage signals the end of prophase I.
Metaphase I
The tetrads line up along the metaphase plate.
Anaphase I
The spindle fibres contract, causing the homologous chromosome pairs to separate.
Telophase I
Two nuclei form.
Cytokinesis
Two daughter cells form as the cytoplasm divides between the newly-formed nuclei. Each daughter cell has a haploid complement of chromosomes.
Meiosis II
Meiosis II follows Meiosis I. The phases of Meiosis II are identical to those of mitosis. After the completion of meiosis II and cytokinesis, four haploid daughter cells are formed.
Genetic variation caused by meiosis
In sexually reproducing species, meiosis causes genetic variation between generations. The three main processes responsible for this are:
- Homologous recombination in Prophase I. Complementary regions of non-sister chromatids undergo double-stranded DNA breaks and are swapped. This results in maternal and paternal alleles being exchanged. One analogy to describe homologous recombination is the shuffling of two decks of playing cards: if two piles of cards are made (e.g. red and black), then the shuffling and combining of these piles brings the red and black cards for each suit together.
- Random segregation of homologous chromosomes in Anaphase I: In Metaphase I, homologous chromosome pairs line up along the metaphase (equatorial) plate. However, for each homologous pair, the maternal and paternal chromosomes line up randomly on either side of the metaphase plate. Thus, during Anaphase I, there is a randomised collection of maternal and paternal homologs moving towards each pole. Consequently, the two haploid daughter cells that form after meiosis I will contain different maternal and paternal chromosome combinations. The random segregation of homologous chromosome pairs is also referred to as the Principle of Independent Assortment. In human cells, there are 223 (~ 8 million) possible combinations of maternal and paternal chromosomes at each meiotic division.
- Random fertilisation of gametes: The gametes produced as a result of meiosis are genetically diverse. During sexual reproduction, one male gamete will fertilise a single female gamete. This process is random; thus, the zygote is genetically distinct from its parents.
Sexual life cycles
Based on the preceding discussion, it is evident that most sexually reproducing have distinct haploid and diploid phases in their life cycle. In organisms such as humans, both the diploid and haploid phases occur within each individual person. However, in other organisms such as fungi and ferns, those phases occupy distinct stages of their life cycles.
